1. Field of the Invention
This invention pertains to radiolocation of mobile vehicles, such as aircraft, with respect to each other within the coverage of a scanning radar at a reference location.
2. Description of the Prior Art
Major airports and way points are presently equipped with secondary surveillance radar (SSR) adapted to cooperate with transponder beacons carried on aircraft to discriminate against interference and ground clutter and to provide for transmission of identification and other data such as altitude from the craft to the groundbased radar. A traffic controller observing the radar display directs the pilots of the involved aircraft by radio, usually with voice communication, so as to maintain or restore safe separations between craft. Such systems are limited in capability because each craft must be dealt with individually and requires its share of the controller's time and attention and its share of the available radio spectrum. When traffic is heavy, takeoffs and landings are delayed, and the possibility of collisions increases.
The number of mid-air collisions and near misses has become so large in busy areas that numerous interaircraft cooperative proximity warning systems have been proposed. Those more prominently under study or development at this time involve frequent or quasicontinuous exchange of signals between all cooperative aircraft within the region of interest and make no provision for non-cooperating aircraft. The required airborne equipment would be bulky and expensive, use more of the already crowded radio spectrum and would be generally independent of other needed and existing equipment, such as transponders. Another drawback of some of the proposed systems is that they provide only relative positional information, without ground reference but in effect with respect to a randomly floating reference.
My copending U.S. patent application Ser. No. 130,952, filed Apr. 5, 1971, now U.S. Pat. No. 3,735,408 and entitled Common Azimuth Sector Indicating System describes the use of a standard airborne transponder with additional equipment including a receiver for receiving other's transponder replies and means for indicating the presence of another .[.transponderequipped.]. .Iadd.transponder-equipped .Iaddend.aircraft within a monitored airspace sector or volume.